The invention relates to the field of transporting and storing high level waste. In particular, the invention relates to a system, method and apparatus for transferring high level waste to and from a spent fuel pool.
In the operation of nuclear reactors, it is necessary to remove fuel assemblies after their energy has been depleted down to a predetermined level for continued reactor operations. Fuel assemblies are typically an assemblage of long, hollow, zircaloy tubes filled with enriched uranium. Upon depletion and subsequent removal from the reactor, spent nuclear fuel is still highly radioactive and produces considerable heat, requiring that great care be taken in its packaging, transporting, and storing. Specifically, spent nuclear fuel emits extremely dangerous neutrons and gamma photons. It is imperative that these neutrons and gamma photons be contained at all times.
In defueling a nuclear reactor, the spent nuclear fuel is removed from the reactor and placed in a canister that is submerged in a spent nuclear fuel pool. The pool facilitates cooling of the spent nuclear fuel and provides radiation shielding in addition to that which is supplied by the canister. Because it is preferable to store spent nuclear fuel in a “dry state,” the canister must eventually be removed from the spent nuclear fuel pool. However, the canister alone does not provide adequate containment of the radiation. As such, apparatus that provide additional radiation shielding during the transport and long-term storage of the spent nuclear fuel are necessary. In state of the art facilities, this additional radiation shielding is achieved by placing the loaded canisters in large cylindrical containers called casks. There are two types of casks used in the industry today, storage casks and transfer casks.
A storage cask is used to store spent nuclear fuel in the “dry state” for long periods of time. Typically, storage casks weigh approximately 150 tons and have a height greater than 15 feet. Storage casks are generally too heavy to be lifted by most nuclear power plant cranes and they are too large to be placed in spent nuclear fuel pools. Thus, in order to store a canister of spent nuclear fuel in a storage cask, the canister must be removed from the pool, prepared in a staging area, and transported to the storage cask.
A transfer cask facilitates removal from the fuel pool and transport of the loaded canister to the storage cask. In facilities utilizing transfer casks to transport loaded canisters, an empty canister is placed into the cavity of an open transfer cask. The canister and transfer cask are both submerged in the spent nuclear fuel pool. As each assembly of spent nuclear fuel is depleted, it is removed from the reactor, lowered into the fuel pool and placed in the submerged canister (which is within the transfer cask). The loaded canister is then fitted with its lid, enclosing the spent nuclear fuel and water from the pool within. The canister and transfer cask are then removed from the pool and set down in a staging area to prepare the spent nuclear fuel for storage in the “dry state.”
The placement of the canister and transfer cask into the fuel pool, loading of the spent nuclear fuel into the transfer cask and the removal of the loaded transfer cask from the fuel pool are carried out by using a high-load capacity overhead crane. FIG. 1 shows a typical high-load capacity overhead crane used for placing cask 7 within fuel pool 4. The crane comprises crane block 11, cables 12, sling 13, extension 10 and yoke 9. Connected to crane block 11 is sling 13 which is connected to extension 10, which is connected to lift yoke 9 that is attached to cask top 8 in order to lift cask 7. Crane block 11 needs to be high enough to allow cask 7 to be lifted over edge 3 of spent fuel pool 4. It is highly desirable that crane block 11, cables 12 and other important crane elements not be immersed in the fuel pool water. If crane block 11 and cables 12 contact the fuel pool water, they will become contaminated. Contamination of the crane block 11 and cables 12 is undesirable because these components are often used outside of the proscribed areas of the nuclear facility. If crane block 11 and cables 12 are contaminated, then it is almost impossible to decontaminate the equipment itself and the grease and oils used for lubricating the equipment. FIG. 2 shows cask 7 fully lowered into fuel pool 4 while crane block 11, cables 12 and sling 13 remain dry. This shows the ideal configuration for cask 7 placement in the fuel pool 4.
A common architectural limitation of nuclear plants pertains to a deep fuel pool wherein the crane bridge is situated at a relatively low elevation above the pool deck. At such plants, placing the heavy transfer cask on the bottom of the fuel pool, i.e. on the fuel pool liner 5, results in the undesirable situation of the crane block 11 and cables 12 being immersed in the pool's contaminated water. Some plants deal with this limitation by making a two-tiered fuel pool having a shallow tier and a deep tier. This allows cask 7 to be lowered in two stages; the first stage using just lift yoke 9 and the second stage using lift yoke 9 with extension 10. The shallow tier serves as a platform for the following changeover procedure: while the crane block 11 is kept at its maximum elevation, cask 7 is placed on the shallow tier, then an extension 10 of suitable length is installed so that the crane block 11 can remain at its maximum elevation while lowering the transfer cask 7 into the deep tier. The extension 10 serves to keep the crane block 11 and cables 12 above the fuel pool water as the transfer cask 7 is picked up from the shallow tier and lowered to the bottom of the deep tier. The reverse procedure is performed when removing the loaded transfer cask from the fuel pool. Creating a two-tiered fuel pool is an inefficient and costly use of the limited space available in nuclear plants because the entire shallow tier is useful only as the surface for the crane parts changeover. Moreover, many sites do not even have the necessary space or structural means to establish a two tiered pool. Other measures, such as wrapping the crane block in plastic are only partially effective in keeping the crane block and cables from becoming contaminated.
Thus, a need exists for providing an effective and cost efficient way to protect the crane block and cables from contamination by the fuel pool water during fuel pool operations in plants having a crane bridge of low elevation and/or a deep fuel pool.